1. Users usually use the feedback option when they want to complain. So an uproar caused due to redesign may not represent view of entire community. Usability principle (bitter truth): its more important to see what users do instead of what they say. You are right when you say people hate changes in their routine. A good litmus test would be to introduce the old design one year down the line for the people who complained
2. The increase in traffic may be because of various reasons. The redesign might have introduced new configurations, apps, screens.
3. A ‘new look facebook’ is more likely to create general buzz for the product as compared to say a twitter buyout. Meaning, more people will talk about it in offices, on lunch and more new people will get introduced to it, that might be the reason of more registrations, and not necessarily the redesign itself.
4. In the same time period, they also introduced the concept of facebook usernames and easy profile URLs. Given the value of premium keywords on facebook, you can’t deny possibility of username hoarding and similar activities.

The catch is to know what is really going to work. Yes, your users may crib, ‘cos everyone resists change (people read “who moved my cheese”, but then don’t know to implement it in life).

The irony is, it could have swung any way. If it had swung the other, then there would have been millions of fingers pointed saying “I told you so”.

I am so glad it didn’t!